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The ultrathin copper sensor is deposited atop a conventional battery separator (white square) as part of an early warning system to alert users that a lithium-ion battery is overheating. (Mark Shwartz, Precourt Institute for Energy, Stanford University)

As the use of lithium-ion batteries continues to expand into millions upon millions of cellphones, laptops, and other electronic devices, their use is growing as well in automotive and aircraft applications.

But not without some setbacks. Prior to more widespread use in the mobility industry, around 2006 Sony recalled millions of lithium-ion batteries after reports of double-digit laptop fires. According to Sony, during the manufacturing process tiny metal impurities had gotten inside the batteries, causing them to short-circuit.

Closer to home and more recent, last year Boeing was forced to temporarily ground the 787 fleet in no small part because battery packs in two airplanes overheated and/or caught fire. Though the packs were redesigned, the cause of the fires has yet to be determined.

That said, Stanford University scientists are working on a "smart" lithium-ion battery that they hope will at least provide ample warning before it overheats.

"Our goal is to create an early-warning system that saves lives and property," said Yi Cui, Associate Professor of Materials Science and Engineering. "The system can detect problems that occur during the normal operation of a battery." Cui added that "normal operation" does not apply to batteries damaged in a collision or other accident.

Cui said that the likelihood of a bad thing such as a battery fire are "maybe one in a million. That's still a big problem, considering that hundreds of millions of computers and cellphones are sold each year. We want to lower the odds of a battery fire to one in a billion or even to zero."

A typical lithium-ion battery consists of two tightly packed electrodes–a carbon anode and a lithium metal-oxide cathode–with an ultrathin polymer separator in between. The separator keeps the electrodes apart. If it's damaged, the battery could short-circuit and ignite the flammable electrolyte solution that shuttles lithium ions back and forth.

The separator is made of the same material used in plastic bottles. It's porous so that lithium ions can flow between the electrodes as the battery charges and discharges.

Manufacturing defects, such as particles of metal and dust, can pierce the separator and trigger shorting, as Sony had discovered. Shorting can also occur if the battery is charged too fast or when the temperature is too low, known as overcharge.

"Overcharging causes lithium ions to get stuck on the anode and pile up, forming chains of lithium metal called dendrites," said Cui. "The dendrites can penetrate the porous separator and eventually make contact with the cathode, causing the battery to short. In the last couple of years we've been thinking about building a smart separator that can detect shorting before the dendrites reach the cathode."

To address the problem, Cui and his colleagues applied a nanolayer of copper onto one side of a polymer separator, creating a novel third electrode halfway between the anode and the cathode.

"The copper layer acts like a sensor that allows you to measure the voltage difference between the anode and the separator," said Densy Zhuo, a graduate student under Cui. "When the dendrites grow long enough to reach the copper coating, the voltage drops to zero. That lets you know that the dendrites have grown halfway across the battery. It's a warning that the battery should be removed before the dendrites reach the cathode and cause a short circuit."

The buildup of dendrites is most likely to occur during charging, not during the discharge phase when the battery is being used.

"You might get a message on your phone telling you that the voltage has dropped to zero, so the battery needs to be replaced," Zhuo said. "That would give you plenty of lead time. But when you see smoke or a fire, you have to shut down immediately. You might not have time to escape. If you wanted to err on the side of being safer, you could put the copper layer closer to the anode. That would let you know even sooner when a battery is likely to fail."

In addition to observing a drop in voltage, researchers were able to pinpoint where the dendrites had punctured the copper conductor simply by measuring the electrical resistance between the separator and the cathode. The location of the tiny puncture holes was confirmed by actually watching the dendrites grow under a microscope.

"The copper coating on the polymer separator is only 50 nanometers thick, about 500 times thinner than the separator itself," said Hui Wu, a postdoctoral fellow in the Cui group. "The coated separator is quite flexible and porous, like a conventional polymer separator, so it has negligible effect on the flow of lithium ions between the cathode and the anode. Adding this thin conducting layer doesn't change the battery's performance, but it can make a huge difference as far as safety."

Most lithium-ion batteries are used in small electronic devices. "But as the electric vehicle market expands and we start to replace onboard electronics on airplanes, this will become a much larger problem," said Zhuo.

"The bigger the battery pack, the more important this becomes," said Cui. "Some electric cars today are equipped with thousands of lithium-ion battery cells. If one battery explodes, the whole pack can potentially explode."

The early-warning technology can also be used in zinc, aluminum, and other metal batteries. "It will work in any battery that would require you to detect a short before it explodes," Cui said.